Problems when trying to prototype RF circuits

Discussion in 'General Electronics Chat' started by Electroguy86, Jun 19, 2013.

  1. Electroguy86

    Thread Starter New Member

    Nov 20, 2012
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    Hello, I am trying to build an RF FM Transmitter that transmits from 88 Mhz- 108 Mhz but I am having a very difficult time doing so. The transmitter I have tested works on a breadboard, which I read is not suppose to due to high capacitance, but for some reason and luck it does work. The problem is when I try and prototype it on a perfboard or anything else the damn thing will not work! The digital tuner part does but the oscillator does not. I have tried the "Manhattan" and "dead bug" construction styles on a PCB with the use of a ground plane and so far it will not work. It will oscillate, as I can sometimes hear on a receiver, but when I hook the PLL up it does not tune or do anything. I am at a loss as to what to do. Is there something I am missing, maybe shorter leads, longer leads, voodoo rituals and animal sacrifices? Or perhaps the oscillator needs to be designed better. I could use some help please. Attached is the schematic. For VT1 I am not using a BF199 I am using SS9018HBU RF transistor and I am aware that the 220k and 470k resistors in the oscillators should be in the ohms and not the kohm range, also thinking that the BFR91 transistor is backwards, corrected on breadboarded circuit and seems to work and for the varicaps I am using MV2109's as that is all I have. If anyone can help that would be awesome.
     
  2. jmsheehan

    New Member

    Mar 31, 2013
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    Bump bump please
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Obviously your oscillator requires large amounts of random capacitance to work.
     
    absf likes this.
  4. absf

    Senior Member

    Dec 29, 2010
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    lol ;)
     
  5. jmsheehan

    New Member

    Mar 31, 2013
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    That's what I figured but how to fix that? Also it should be noted that I did not design this oscillator, got it from some website and had the parts do I figured I would try and build it.. Cause why not, seemed like fun. Also, I'm signed into a different username. Could not remember password or email to electroguy86.
     
  6. Electroguy86

    Thread Starter New Member

    Nov 20, 2012
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    So, does anyone know what I can do?
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Paragraphs please!
     
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  8. upopads

    Active Member

    Dec 18, 2007
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    BFR91 is NPN obsolete transistor. Can you talk about why you chose the transistor topology you did for the signal output. BFF 199 does seem to be incorrect but I dont know this
     
  9. absf

    Senior Member

    Dec 29, 2010
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    What do you mean you connected it backwards. Did you connect the emitter to V+ and collector to GND? But in the circuit BFR91 is drawn as PNP but it is actually NPN. See P4 on the table with small prints as datasheet attached.

    Allen
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I think you're not getting much response because there are just too many variables in prototyping RF. It could be anything from parasitic inductance and capacitance to a simple error in wiring. If it were me, I'd prototype it a stage at a time, verifying operation of each stage as I go. That's how this kind of prototyping is usually done.
     
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